1. Field of the Invention
The present invention relates to an apparatus for processing a rubber and a processing method therefor.
2. Description of the Related Art
In general, a rubber is processed in the order of mixing, rolling/extruding and forming. Among these, the mixing step includes mastication step and kneading step. The kneading step is conducted for the purpose of mixing and dispersing compounding agents in raw rubber finely and homogenously. On the other hand, the mastication step is conducted prior to the kneading step for softening and homogenizing the rubber itself such that the compounding agents can easily be mixed and dispersed in the raw rubber in the subsequent kneading step.
The kneading step is generally separated into two steps, namely A-kneading and B-kneading. A-kneading is a step of adding an agent and kneading the mixture, in which the agent contains a reinforcing agent such as silica and carbon black, but excludes vulcanization components (e.g., vulcanizing agent and vulcanization accelerator). B-kneading is a step of adding vulcanization components to the material obtained by the A-kneading step and then kneading the mixture.
For any step of the mastication, A-kneading and B-kneading, temperature control plays a critical role. (References are made to, for example, Japanese Unexamined Patent Publication (Kokai) Hei 7-227845, Japanese Unexamined Patent Publication (Kokai) No. 2004-352831, Japanese Unexamined Patent Publication (Kokai) No. 2007-8112 and Japanese Unexamined Patent Publication (Kokai) No. 2007-320184).
(1) First, a description is made on the mastication step. Mastication serves for exerting shear force on a rubber material to cut its molecular chains, to reduce its viscosity and to impart plasticity to the material for forming. There are two types of mastication, i.e., low temperature mastication and high temperature mastication. As described in Japanese Unexamined Patent Publication (Kokai) Hei 7-227845, a peptizer (i.e., a mastication promoter) has conventionally been added during the kneading of the high temperature mastication.
Low temperature mastication is mechanical mastication conducted at a temperature up to about 110° C. In this mastication step, the longer portions of molecular chains are selectively cut to decrease the molecular weight. On the other hand, high temperature mastication is chemical mastication conducted at a higher temperature, for example, from 120° C. to 180° C., with an addition of a peptizer to cause a chemical reaction and thus to cut the molecular chains.
The mastication of natural rubber is conducted in an enclosed-type mixer (e.g., a Banbury mixer). Heat is generated as the kneading of the rubber proceeds, increasing the temperature of the rubber. Particularly as in the case of high temperature mastication, holding the rubber at a high temperature, for example, from 150° C. to 180° C., within an enclosed space for an extended period of time may cause the rubber to burn (or to scorch). Therefore, a general practice is to add a peptizer to impart plasticity to the raw rubber at an early step and thus to shorten the mastication time.
In the case of the high temperature mastication using a peptizer, a molecular chain is cut, regardless of its molecular weight, at a reaction point of the peptizer. As a result, various compounds with small to large molecular weights are considered remain in the rubber composition after the mastication. This may lead to segregation of the peptizer and may further increase the non-uniformity of the reaction.
On the other hand, as described above, mechanical mastication conducted at a low temperature selectively cuts the portions with larger molecular weights, which evenly reduces the weights of molecules in the rubber compound. Therefore, mechanical mastication is more advantageous in stabilizing the chemical properties of the rubber after the mastication.
From the above point of view, the mastication step is preferably conducted solely as mechanical mastication. In a conventional method, an attempt to solely conduct mechanical mastication faces a problem that the heat, associated with the kneading of rubber, increases the temperature of the rubber. In other words, the temperature rises with the progress of the mastication and exceeds the temperature range suitable for the mechanical mastication with imparting sufficient plasticity to the rubber, causing the rubber to scorch. With this background, there is a need for a technique that enables mastication while maintaining a most suitable temperature.
(2) Next, a description is made on A-kneading. In recent years, silica has been used instead of carbon black as a reinforcing agent to be added during the A-kneading step. Silica, however, has a drawback that its ability to reinforce rubber is smaller than that of carbon black. Therefore, when mixing silica into a rubber component, a silane coupling agent is added to increase the reinforcing ability. When added as a compounding agent, the silane coupling agent is fixed on the silica by its coupling reaction and, furthermore, reacts with the rubber component, increasing the dispersibility of the silica in the rubber and improving the reinforcing ability.
It has been known that, in order to make the silica react with the silane coupling agent efficiently, the kneading is desirably conducted at a specific temperature for an extended period of time. Under a low temperature environment, a coupling reaction is less likely to occur between the silica and a silane coupling agent. On the other hand, an excessively high temperature causes the rubber to cross-link, which rapidly increases the viscosity and causes a gel to be formed.
When a rubber component is kneaded with silica and a silane coupling agent in a mixer, the interior temperature of the mixer rises due to the heat generated by the viscous flow of the rubber and the heat associated with the coupling reaction. As a result, the temperature reaches a level at which a gel is formed within a short period of time, and thus causing a problem that not enough time is allowed for the coupling reaction.
Japanese Unexamined Patent Publication (Kokai) No. 2004-352831 discloses a method in which kneading step before the addition of silane coupling agent is conducted using an enclosed-type Banbury mixer, while the coupling reaction after the addition of the silane coupling agent is conducted using a separate mixer that is not enclosed. Also, Japanese Unexamined Patent Publication (Kokai) No. 2007-8112 discloses a method in which a rubber composition that has been kneaded is transferred onto a pair of kneading rolls at a temperature slightly below the temperature range suitable for the coupling reaction, wherein the shear force exerted by the rolls causes the self-generation of heat for the coupling reaction.
However, the methods of Japanese Unexamined Patent Publication (Kokai) No. 2004-352831 and Japanese Unexamined Patent Publication (Kokai) No. 2007-8112 require mixers and rolls dedicated to coupling reactions. These Documents do not disclose a technique for kneading within a suitable temperature range without increasing the installation space.
(3) Next, a description is made on B-kneading. As described in Japanese Unexamined Patent Publication (Kokai) No. 2007-320184, B-kneading step is generally conducted at a temperature lower than the temperature at which A-kneading step is conducted. The following is the reason. During the B-kneading step, vulcanization components are added as additives. Therefore, when the material is kept under a high temperature, the material temperature may become excessively high during the kneading, which may cause the rubber composition, still insufficiently kneaded, to cross-link. Once such a phenomenon occurs, the hardness of the rubber increases and the formability deteriorates. Furthermore, in the case where insoluble sulfur is used as a vulcanization ingredient, the sulfur may be transformed into soluble at a high temperature, resulting in the formation of a bloom which causes adhesive failure.
When rubber components are kneaded in a mixer, the interior temperature of the mixer rises due, for example, to the heat generated by viscous flow of the rubber. Especially when the kneading is conducted with vulcanization components added, crosslinking occurs as soon as the temperature reaches about 120° C. Therefore, in a conventional practice, the content is discharged from the mixer as soon as this temperature is reached. In this case, however, the vulcanizing agents may not be kneaded for a sufficient period of time, causing the vulcanizing agents to be poorly dispersed. A rubber composition having vulcanizing agents poorly dispersed is later formed into a product containing foreign matters consisting of vulcanization accelerators such as a guanidine compound. In addition, the finished product may have a problem of multiple failures, such as non-uniform surface color and generation of internal pores, which increase the rejection rate.
Japanese Unexamined Patent Publication (Kokai) No. 2007-320184 discloses a construction having a cooling apparatus for lowering the internal temperature of a mixer. Such a cooling apparatus not only increases the installation space, but also increases the production cost significantly. Furthermore, the interior temperature of the mixer is largely affected by the performance and functions of the cooling apparatus and may not necessarily be maintained in a desirable condition at all times.